In coal mining, it has been common practice to apply limestone in the form of a dust to the walls of a mine, thereby causing the limestone to adhere to the walls. The process, known as “rock dusting,” has two effects. First, because the limestone dust covers exposed surfaces of unmined coal, it prevents mine fires from being propagated along those exposed surfaces. Second, if methane, coal dust, or a mixture of methane and coal dust, ignite in a mine causing an explosion, the rock dust adhering to the mine wall will become airborne, and suppress the propagation of fire resulting from the explosion.
The United States Mine Safety and Health Administration has established standards for rock dusting, which include a requirement that all exposed surfaces of a mine be covered with rock dust at least 80% of the content of which is non-combustible. Existing methods for applying rock dust include application of rock dust to a mine wall. Recently, mines have begun using chemical foam to achieve improved adhesion of the rock dust to mine surfaces. One method of using foam in rock dust application is to apply a dry mixture of rock dust and a foaming agent to a mine wall. Another method is to apply a mixture of foam and rock dust to a mine wall. In the last-mentioned method, the foam is formed, mixed with rock dust in a mixing vessel, and pumped through a conduit to the point of application. A system for utilizing foam to enhance the adhesion of rock dust to a mine wall is described in U.S. Pat. No. 6,726,849, granted Apr. 27, 2004. Still another method, described in United Stated Patent Publication 2012/0181051, published Jul. 19, 2012, utilizes a composition made up of rock dust, water, a pumping aid and a polymer is applied to a mine wall. The pumping aid can be a foaming agent such as a laurel ether sulfate. A foamed slurry composition is prepared by mixing rock dust and water in a tank, using a paddle mixer in the bottom of the tank for agitation. The pumping aid is added to the slurry, and a foamed slurry is formed by further agitation by the paddle mixer. The foamed slurry is then pumped to a nozzle for spraying.
In known systems in which a foam is combined with rock dust upstream of a delivery pump, or in which a mixture including rock dust and a foaming agent is caused to foam upstream of the pump, several problems can arise. The expansibility of the gas component of the foam can either interfere with the operation of the pump or limit the proportions of ingredients in the foamed compositions to ranges that allow the composition to be pumped effectively. In addition, the action of the pump can cause deterioration of the foam, requiring the generation of large amounts of foam, or the use of large amounts of foaming agent, to provide adequate quantities of foam in the mixture being applied to the mine wall.
In still another method, described in my copending U.S. patent application Ser. No. 13/736,112, filed Jan. 8, 2013, an apparatus for applying rock dust to a mine wall comprises first and second conduits. Rock dust is entrained in air in the first conduit, and a flowable foam, made by mixing a foamable liquid and air, is delivered through the second conduit. Rock dust and air taken from the first conduit are combined with flowable foam taken from the second conduit, and the combined mixture of air, rock dust and foam is applied by a nozzle to a mine wall.
Among the apparatus and methods used previously for applying rock dust, the method described in U.S. patent application Ser. No. 13/736,112 has, among its advantages, the ability to combine rock dust and foam near the point of application to the mine wall, not only avoiding difficulties encountered in pumping a foamed mixture, but also avoiding the deterioration of foam that takes place when a foam travels through a long conduit along with rock dust. However, the air used to entrain the dust in the process of patent application Ser. No. 13/736,112 also causes the foam to deteriorate, even though the foam is combined with the air-entrained rock dust at the location of the nozzle.